I have a 15 and a 13 year old.
I have only checked the 15 year olds phone once, he was about 13. I had it reported back to me via someone else that he had some nasty stuff on his instagram. We did similar to PP.
Give me phone now, or contract is cancelled. He handed phone over.
He also went ballistic, as he didn't want me reading all his messages etc.
I was very clear. I wasn't interested in most content, I was skimming through the whole to check if there was anything that shoudln't be. I wasn't going to read his texts etc between him and his friends.
Don;t know if he believed me really, but I stuck to that. I did check his whole instagram, and it was fine.
Then we talked about it and he admitted that he was following someone who posted vile stuff, if it was vile, he instantly deleted. He unfollowed them, and we returned phone. I was very clear that I had no intention of micro managing, until I thought there was a problem, and then I reserved the right to check.
It is most likely that he has posted private messages to a girl, or that he has posted stuff he wouldn't want you to read, but not actually 'bad'